The term "reversal photographic element" designates a photographic element which produces a photographic image for viewing by being imagewise exposed and developed to produce a negative of the image to be viewed, followed by uniform exposure and/or fogging of residual silver halide and processing to produce a second, viewable image. Color slides, such as those produced from Kodachrome.TM. and Ektachrome.TM. films, constitute a popular example of reversal photographic elements. In the overwhelming majority of applications the first image is negative and the second image is positive.
Although tabular grains had been observed in silver bromide and bromoiodide photographic emulsions dating from the earliest observations of magnified grains and grain replicas, it was not until the early 1980's that photographic advantages, such as improved speed-granularity relationships, more rapid developability, increased thermal stability, increased separation of blue and minus blue imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats, were realized to be attainable from silver halide emulsions in which the majority of the total grain population based on grain projected area is accounted for by tabular grains satisfying the mean tabularity (T) relationship: EQU D/t.sup.2 &gt;25
where
D is the equivalent circular diameter (ECD) in micrometers of the tabular grains and
t is the thickness in micrometers of the tabular grains.
Once photographic advantages were demonstrated with tabular grain silver bromide and bromoiodide emulsions techniques were devised to prepare tabular grains containing silver chloride alone or in combination with other silver halides.
Notwithstanding the many established advantages of tabular grain emulsions, the art has observed that these emulsions tend toward more disperse grain populations than can be achieved in the preparation of regular, untwinned grain populations--e.g., cubes, octahedra and cubo-octahedral grains. This has been a concern in some, but not all, photographic applications for tabular grain emulsions.
In the earliest tabular grain emulsions dispersity concerns were largely focused on the presence of significant populations of nonconforming grain shapes among the tabular grains conforming to the aim grain structure. While the presence of nonconforming grain shapes in tabular grain emulsions has continued to detract from achieving narrow grain dispersities, as procedures for preparing tabular grains have been improved to reduce the inadvertent inclusion of nonconforming grain shapes, interest has increased in reducing the dispersity of the tabular grains.
The following are illustrative of tabular grain emulsions with relatively high levels of grain uniformity:
R-1 Research Disclosure, Vol. 232, August 1983, Item 23212 (Mignot French Patent 2,534,036, corresponding) discloses the preparation of silver bromide tabular grain emulsions with coefficients of variation ranging down to 15. Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley Annex, 21a North Street, Emsworth, Hampshire P010 7DQ, England. The tabular grain emulsion preparation technique has not been attractive for large scale emulsion manufacture because of the extended grain ripening periods required. The emulsion preparation process is further limited in that is applicable only to the preparation of silver bromide emulsions.
R-2 Saitou et al U.S. Pat. No. 4,797,354 reports in Example 9 a silver bromide tabular grain emulsion containing a relatively high proportion of hexagonal tabular grains. Low levels of grain dispersities have been demonstrated only in the preparation of silver bromide emulsions.
The following are illustrative of the incorporation of tabular grain emulsions in multicolor photographic elements:
R-3 Kofron et al U.S. Pat. No. 4,439,520 illustrates the incorporation of high aspect ratio (D/t&gt;8) tabular grain emulsions in multicolor photographic elements. Kofron et al reports improved speed-granularity relationships, increased image sharpness and reduced blue contamination of minus blue (green and/or red) records.
R-4 Sowinski et al U.S. Pat. No. 4,656,122, which specifically addresses color reversal photographic elements, has reported increased threshold imaging speeds, reduced toe region density, increased maximum density and increased contrast to result from blending a smaller grain emulsion with a tabular grain emulsion, thereby increasing the overall dispersity of the resulting emulsion.
Only limited use of high tabularity emulsions in color reversal photographic elements has occurred prior to the present invention. The result has been color photographic elements with higher levels of image granularity, lower dye image contrast and lower levels of image sharpness than optimum.
Interestingly, the most common multicolor photographic element format coats a yellow dye image forming layer unit over each of magenta and cyan dye image forming layer units with nontabular grain emulsions being employed in the yellow dye image forming layer unit. This is Layer Order Arrangement I of Kofron et al, and, as clearly taught by Kofron et al, the presence of nontabular grains in an overlying dye image forming layer unit degrades the sharpness of the dye image obtained in each of the underlying magenta and cyan dye image forming layer units.